The present invention relates to a fail-safe control for an electronically controlled automatic transmission to be mounted on a vehicle, and, more particularly, to a fail-safe control against a failure of a solenoid for controlling a lockup in an electronically controlled automatic transmission.
In a conventional electronically controlled automatic transmission, changes in gear ratio of the transmission (hereinafter ("T/M") and the operation of the lock-up clutch are, in general, effected in response to signals representing the degree of opening of the throttle and the car speed by operation of an actuator such as a solenoid secured to the T/M is controlled.
In a case where an actuator such as a solenoid for controlling lockup (hereinafter "lock-up solenoid") experiences a problem such as short circuit or a disconnection, the lock-up clutch is generally turned off responsive to the failure of the actuator.
However, in the conventional control method, since the lockup operation is always prohibited during the above-described type of failure, a problem arises that a slippage occuring in the torque converter causes a rise of the oil temperature of the T/M when operation load is high, as at the time of high-speed running at the maximum gear stage (the fourth gear in general) or on a mountain drive or the like.